The present invention relates to the production of silicon tetrafluoride gas, and especially to the production of such gas by reaction between concentrated sulfuric acid and aqueous fluosilicic acid.
Silicon tetrafluoride gas is an article of commerce useful for a variety of purposes, and especially as a chemical intermediate; most notably to make fumed silica (see U.S. Pat. No. 3,969,485 of Flemmert and the references cited therein) and to make silane (see U.S. Pat. No. 4,374,111 of Lefrancois). Generation of silicon tetrafluoride by mixing concentrated sulfuric acid with aqueous fluosilicic acid is disclosed in the above-referenced Flemmert patent, as well as in other references including U.S. Pat. Nos. 3,218,124 to Oakley, Jr et al. (1965) and 4,062,930 to Zawadzki et al. (1977). The general scheme used in these references is a stirred tank, into which both liquids are fed, and a column in which the product gas is scrubbed with concentrated sulfuric acid. The references differ as to whether the sulfuric acid passing through the column represents a portion or all of the concentrated sulfuric acid stream fed to the reactor. Certain of these references provide for silica (sand) feed to the reactor. Without any silica feed, the net reaction is ##STR1## With silica fed in the proper amount, the net reaction is ##STR2## It is indicated that the by-product acid should have a concentration of about 72-78%. U.S. Pat. No. 4,062,930, col. 2, lines 29-32. In such processes, depending upon the reaction temperature and other factors (including silica feed) the by-product acid will have various levels of fluoride (as dissolved HF and/or dissolved SiF.sub.4) and silicon (as SiF.sub.4) values.